The Boeing Company 

2401 E. Wardlow Road 

Long Beach, CA 90807-5309 

______________________________________________________________________________________________________ 

May 21, 2015

CP-L4L-15-00191

U.S. Department of Transportation

Docket Operations M-30

1200 New Jersey Avenue, SE

Room W12-140, West Bldg. Ground Floor

Washington DC 20590-0001

Attention: FAA Transport Airplane Directorate

Applicant: The Boeing Company

Boeing Commercial Airplanes

Southern California Engineering Design Center

2401 E. Wardlow Road, Mail Code D800-0022

Long Beach, CA 90807-5309

Contact: Mr. Thomas A. Enyart

Email: Thomas.a.Enyart@boeing.com

Phone: (562) 797-8956

Seal Beach, CA 90740-1515

Subject: Petition for Exemption from 14 CFR 25.901(c) Amendment 25-126 and 14

CFR 25.981(a)(3) Amendment 25-125 pertaining to planned changes for 

the 757-200 (Freighters Only), 757-200PF Center Wing Tank Fuel

Quantity Indication System (FQIS) Fuselage Wiring Installation.

Reference: a) NPRM 2011-NM-094-AD

b) NTSB report TWA 800 AAR-00-03

c) Service Bulletin 757-28-0136 BITE check

Certification Item No: 48065

The Boeing Company respectfully submits the enclosed Petition for Exemption from the Title 

14 Code of Federal Regulations §14 CFR 25.901(c) Amendment 25-126 and 14 CFR 

25.981(a)(3) Amendment 25-125 pertaining to planned changes for the 757-200 (Freighters 

Only), 757-200PF Center Wing Tank (CWT) Fuel Quantity Indication System (FQIS) 

Fuselage Wiring Installation.

The Extent of Relief Sought and Reason(s):

14 CFR 25.901(c) at Amendment 25-126 requires:

(c) For each powerplant and auxiliary power unit installation, it must be

established that no single failure or malfunction or probable combination of 

failures will jeopardize the safe operation of the airplane except that the
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failure of structural elements need not be considered if the probability of

such failure is extremely remote.

14 CFR 25.901(c) Amendment 25-126 Boeing relief is necessary because: Direct 

compliance would require excessive design changes and retrofit of in-service airplanes. 

Changes include the following: Further redesign of the Airplane Wiring System to 

ensure FQIS wire separation for numerous airplane configurations. Make new tank 

penetrations and procure new in-tank wiring harnesses. Modify the Fuel Quantity 

Processor Unit (FQPU) and associated connectors. The solution would require a FQIS 

architecture change to relocate the FQPU and consider Remote Data Connectors 

(RDCs). In-tank component changes (new brackets, potential need for shorter probes, 

revised in-tank wire harness and retention means). Software changes related to Fuel 

Tank Gauging Function (to account for capacitance changes if shorter in-tank probes 

are required).

14 CFR 25.981(a)(3) at Amendment 25-125 requires:

(a) No ignition source may be present at each point in the fuel tank or fuel 

tank system where catastrophic failure could occur due to ignition of fuel or 

vapors. This must be shown by:

(3) Demonstrating that an ignition source could not result from each 

single failure, from each single failure in combination with each latent

failure condition not shown to be extremely remote, and from all 

combinations of failures not shown to be extremely improbable. The

effects of manufacturing variability, aging, wear, corrosion, and likely 

damage must be considered.

14 CFR 25.981(a)(3) Amendment 25-125 Boeing relief is necessary because: Same as above 

Background

The FAA has issued NPRM 2011-NM-094-AD which proposes to require modifying the 

FQIS wiring or fuel tank systems to prevent development of an ignition source inside the 

center fuel tank. Paragraph (h)(2) states : “Within 72 months after the effective date of 

this AD, modify the airplane by separating FQIS wiring that runs between the FQIS 

processor and the center fuel tank, including any circuits that might pass through a main 

fuel tank, from other airplane wiring that is not intrinsically safe.” 

Boeing is proposing to accomplish the FAA’s planned AD mandated changes to 

separate the FQIS CWT wiring between the FQIS processor and the center fuel tank, as 

well as using the FQIS Built in Test Equipment (BITE) check. The BITE service bulletin 

757-28-0136 is already approved by the FAA. The CWT FQIS wiring separation is 

being prepared with guidance from the FAA’s approved standard for retrofit wire 

separation from FQIS wiring. But the combination of BITE and wire separation will not 

result in the FQIS system being compliant to 25.901(c) [25-126] and 25.981 (a)(3) [25-

125]. Since the wiring changes are a change to type design, but will not be compliant, 

an exemption is being petitioned. FAA’s response to the reference a) NPRM comment 

suggest that FAA will accept petitions for exemption for the “latent-plus-one” 

requirements of sections 25.901(c) and 25.981(a)(3) of the Federal Aviation 

Regulations.
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This petition for an exemption is being made since full compliance to these rules for the 

757 would require extensive system design/installation changes, but do not substantially 

improve the safety of the current airplane design, and are economically prohibitive given 

that the 757 freighter fleet is limited, and so are not in the public’s interest.

The FQIS system was previously shown compliant to §25.1309 and 25.901(c) for TC 

and ATC; however, both §25.901(c) [25-126] and §25.981(a)(3) [25-125] have 

additional failure combination requirements. For systems that may cause a catastrophic 

hazard, §25.1309 requires no single failure and no combination of failures greater than

extremely improbable; §25.901(c) [25-126] requires no single failure or probable

combination of failures; and §25.981(a)(3) [25-125] requires no single failure, no single 

plus latent (greater than extremely remote), and no multiple combination greater than 

extremely improbable.

This exemption is being sought for the no single plus latent (greater than extremely 

remote) as is stated in §25.981(a)(3) and interpreted by the FAA for §25.901(c). An

exemption is also being sought for the non-environmental aspects of §25.901(c) 

provided by FAA interpretation, and the explicit §25.981(a)(3) requirements of 

manufacturing variability, aging, wear, corrosion, and likely damage, as these

considerations are already covered by airline maintenance of the type design of the 

airplane.

Statement of Public Interest:

Without exemption relief for the FQIS CWT wiring changes, the intent of the NPRM 

and the FAA’s previous communication of intent for airplane design improvement for 

the CWT FQIS to address those 757 freighters that do not have Nitrogen Generating 

System (NGS) cannot be met. For compliance, airplane changes to the entire FQIS

(all tanks) would be required for fleet retrofit such as:

x New system architecture to redesign the FQPU to function as a Hot Short 

Protector (HSP*) Line Replaceable Unit (LRU) and relocate the FQPU to enable 

wire separation. Other architectures could be used such as providing a HSP 

LRU closer to the tank, but it would be a more extensive and new architecture.

x Extensive airplane wiring changes to ensure adequate wire separation

throughout the fuel quantity indication system.

x In-tank component changes (new brackets, potential need for shorter probes, 

revised in-tank wire harness and retention means).

x Software changes related to Fuel Tank Gauging Function and Safety function if

shorter probes are required.

* A HSP is an LRU that is also designed to have the function to protect the tank circuit wiring from Electrical

Power Source threats that could affect the airplane circuit wiring of the LRU (FQPU).

The current FQIS system has been thoroughly evaluated for safety aspects as was 

required per SFAR88 for the Fuel Quantity Indicating System, however the FAA 

determined there were safety issues identified with FQIS wiring connected into a high 

flammability tank, for the 757 it is the CWT. The time needed to successfully design 

and implement system changes beyond the proposed CWT FQIS wiring separation 

design changes to comply with the rules would further impact the availability of the 

wiring change on this out of production fleet of 757 aircraft. These new complete 
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FQIS design changes would not provide any economic benefit to the operators and do 

not provide any significant benefit or increased level of safety. The Boeing Company 

considers that the granting of this exemption would negate the need for FAA to 

evaluate the additional data required to support compliance with these regulations. 

Therefore, granting this exemption would reduce the burden on FAA resources and 

consequently public expenditure. 

Granting this exemption is in the public interest for the aforementioned reasons and will:

1. Enhance the safety of the 757 fleet (as has been determined by the FAA) for those

airplanes not installing NGS by adding wiring changes for the CWT FQIS.

2. Enhance the safety of the 757 fleet, by also including FQIS BITE interval checks for

the CWT so that when done in addition with the CWT FQIS wiring separation, the

airplane addresses the single plus latent failure modes for the FQIS which was the

probable failure combination indicated by the NTSB

Statement of No Adverse Effect on Safety.

The addition of the FQIS CWT wiring changes along with a BITE interval check, in lieu 

of a fully compliant (all tanks) FQIS (which would have a HSP FQPU LRU, wire 

separation for the tank circuit wiring (both outside and inside the tank), and a fault 

detecting BITE that is annunciated), does not have an adverse effect on safety. 

In the proposed CWT FQIS wire separation design change, the ignition source threat 

from other airplane systems is reduced since the wiring will have increased separation. 

There is also Instructions for Continued Airworthiness (ICA) that ensures that any other 

system changes will continue to maintain the separation provided by this proposed 

design change which will be now clearly be visually identified as FQIS wiring.

Together, this improves the wiring failure modes, and improves the system probability 

relative to fuel tank ignition. With the addition of the FQIS BITE interval checks for the 

CWT the probability is better since the exposure of wiring failures will be further 

reduced.

For the CWT FQIS, where the changes to the FQIS are being done instead of the 

installation of NGS as determined by the FAA to be an acceptable substitute safety 

improvement, the combination of the separation, the ICA, and the BITE addresses no

single failures in combination with latent failures with an acceptable level of safety by 

ensuring the continued integrity of protection features from threats to the tank circuit 

wiring. 

x A - The fault modes of the in tank wiring have reduced exposure to 750 hours 

due to the BITE check. Wiring inside the tank is well protected and has a very 

low failure probability which is improved by the lower exposure.

x B1 - The FQPU is a HSP up to 28V. If a low probability higher voltage power 

source short were to occur on the airplane side of the FQPU or within the 

FQPU, it most likely would, but for specific failures may not stop it from 

propagating through to the tank circuits, but the failure would at the least be 

detectable when the BITE interval check was done, and in most cases sooner,

as it would most likely affect indication..

x B2 - The wiring from the FQPU to the tank will have separation provided by the 

AD mandated SB. This wiring has low probability failure modes which also 

have reduced exposure provided by the 750 hour interval BITE check.
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Whether the failure is A, or B1, or B2, there is no single failure. For the combination 

of failures, A and B1, or A and B2, the BITE has reduced the exposure and the 

latency interval for each protection design feature (wiring has less than extremely 

remote failure rates) and so the probability for each failure mode is better than 

remote. This also addresses the single plus latent failure requirement with an 

acceptable level of safety as the probability of either failure on the flight before the 

next interval check would both be better than remote.

The 757 FQIS was found to be compliant to §25.901(c) during airplane type certification 

for the FQIS change in 1992 and since then Boeing has completed the SFAR88 safety 

assessment and has increased the airplane fuel system safety with installation of a Hot 

Short Protector (HSP) LRU for the densitometer.

Following the SFAR88 system safety assessments, Boeing assessment was that the 

risks for the 757 FQIS installation were extremely improbable, which is the typical 

industry and CFR §25.1309 standard for an acceptable safety for systems.

The Boeing approach to incorporate design changes to add center wing tank FQIS wire 

separation with visual identification, a CWT FQIS 750 hour BITE check, along with ICA 

that ensures that added systems wiring will maintain the separation for the life of the 

airplane, and the densitometer tank circuit HSP provided by Boeing SB 757-28A0085,

will provide for an enhanced safety margin over the existing, approved FQIS on the inservice fleet.

Request to Waive Publication and Comment:

Since the NPRM 2011-NM-094-AD is directly connected to this change, makes mention 

of this petition for exemption, and has already gone through public commenting 

process, Boeing respectfully requests that the FAA waive the “publication and 

comment” step in the process for making a final decision on this exemption.

Additionally, given that the FAA and Boeing have made a rapid schedule for this project, 

and so the schedule-sensitive nature of this request for exemption, Boeing respectfully 

requests that the FAA waive the “publication and comment” step in the process for 

making a final decision on this exemption. In addition to the timeliness concern, it is the

Boeing position that the safety-associated aspects related to this system have been fully

vetted in the public forum as part of the SFAR88 project.

Privileges of the Exemption Outside the United States:

Per 14 CFR 11.81(h), Boeing requests that the privileges of this exemption be extended 

outside the United States. This extension of privileges is necessary for operations 

based within foreign countries having bilateral agreements with the United States 

accepting FAA 14 CFR part 25 as their airworthiness standards for transport category 

aircraft.

Justification for Exemption:

In accordance to the requirements of 14 CFR §11.81(e), the Boeing company has 

provided a means of compliance to the FAA to establish an acceptable level of safety to 

that provided by the rules 14 CFR §25.901(c) and 14 CFR §25.981(a)(3) by separating 

the FQIS wiring from other airplane power wires that will meet the minimum separation 

requirements for most of the wire runs. For the areas that minimum separation are not 

achievable due to space limitation, other positive separation means, such as clamps, 
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spacers and sleeveing will be used for the CWT FQIS and also including FQIS BITE 

interval checks for the CWT . These design changes address the single plus latent 

failure modes for the FQIS which was the probable failure combination indicated by the 

NTSB report, reference b). Therefore, granting this exemption would, in turn, reduce 

the burden on FAA resources and consequently public expenditure

Conclusion:

Boeing is petitioning for an exemption from the provisions of 14 CFR 25.901(c)[25-126] 

and 14 CFR 25.981(a)(3)[25-125] at the system level as they apply to the FQIS installed 

on the 757-200 (freighters), 757-200PF airplanes for the FQIS wiring changes described 

above.

Sincerely,

Tim Hendrix

Manager, Certification & Airworthiness

Southern California Engineering Design Center

Boeing Commercial Airplanes

(562) 593-7074, MC D800-0022

TE/DP:ap